It has previously been proposed to use so-called spatial light modulators to control the routing of light beams within an optical system, for instance from selected ones of a number of input optical fibres to selected ones of output fibres.
Optical systems are subject to performance impairments resulting from aberrations, phase distortions and component misalignment. An example is a multiway fibre connector, which although conceptually simple can often be a critical source of system failure or insertion loss due to the very tight alignment tolerances for optical fibres, especially for single-mode optical fibres. Every time a fibre connector is connected, it may provide a different alignment error. Another example is an optical switch in which aberrations, phase distortions and component misalignments result in poor optical coupling efficiency into the intended output optical fibres. This in turn may lead to high insertion loss. The aberrated propagating waves may diffract into intensity fluctuations creating significant unwanted coupling of light into other output optical fibres, leading to levels of crosstalk that impede operation. In some cases, particularly where long path lengths are involved, the component misalignment may occur due to ageing or temperature effects.
Some prior systems seek to meet such problems by use of expensive components. For example in a communications context, known free-space wavelength multiplexers and demultiplexers use expensive thermally stable opto-mechanics to cope with the problems associated with long path lengths.
Certain optical systems have a requirement for reconfigurability. Such reconfigurable systems include optical switches, add/drop multiplexers and other optical routing systems where the mapping of signals from input ports to output ports is dynamic. In such systems the path-dependent losses, aberrations and phase distortions encountered by optical beams may vary from beam to beam according to the route taken by the beam through the system. Therefore the path-dependent loss, aberrations and phase distortions may vary for each input beam or as a function of the required output port.
The prior art does not adequately address this situation.
Other optical systems are static in terms of input/output configuration. In such systems, effects such as assembly errors, manufacturing tolerances in the optics and also changes in the system behaviour due to temperature and ageing, create the desirability for dynamic direction control, aberration correction, phase distortion compensation or misalignment compensation.
It should be noted that the features of dynamic direction control, phase distortion compensation and misalignment control are not restricted to systems using input beams coming from optical fibres. Such features may also be advantageous in a reconfigurable optical system. Another static system in which dynamic control of phase distortion, direction and (relative) misalignment would be advantageous is one in which the quality and/or position of the input beams is time-varying.
Often the input and output beams for optical systems contain a multiplex of many optical signals at different wavelengths, and these signals may need to be separated and adaptively and individually processed inside the system. Sometimes, although the net aim of a system is not to separate optical signals according to their wavelength and then treat them separately, to do so increases the wavelength range of the system as a whole. Where this separation is effected, it is often advantageous for the device used to route each channel to have a low insertion loss and to operate quickly.
It is an aim of some aspects of the present invention at least partly to mitigate difficulties of the prior art.
It is desirable for certain applications that a method or device for addressing these issues should be polarisation-independent, or have low polarisation-dependence.
SLMs have been proposed for use as adaptive optical components in the field of astronomical devices, for example as wavefront correctors. In this field of activity, the constraints are different to the present field—for example in communication and like devices, the need for consistent performance is paramount if data is to be passed without errors. Communication and like devices are desirably inexpensive, and desirably inhabit and successfully operate in environments that are not closely controlled. By contrast, astronomical devices may be used in conditions more akin to laboratory conditions, and cost constraints are less pressing. Astronomical devices are unlikely to need to select successive routings of light within a system, and variations in performance may be acceptable.